This invention was made with government support under grant/contract DMR 9415358 awarded by the National Science Foundation. The government has certain rights in the invention.
This invention relates generally to the field of techniques for manufacturing semiconductor substrates which are photoluminescing. More particularly, the invention relates to the manufacture of photoluminescing semiconductor materials, and in particular photoluminescing porous silicon, using laser treatment.
Silicon is widely used in the manufacture of micro-electronic devices, but conventionally produced silicon and other semiconductor materials are not suitable for light emitting diodes or lasers since they do not photoluminesce. It has recently been discovered that essentially porous silicon is photoluminescing and can therefore be utilized for light emitting devices. Porous silicon is generally characterized as an array of free-standing, crystalline silicon columns ranging from 2 to 100 nanometers in diameter with a large surface-to-volume ratio. Pore size and pore density vary.
It is well known to create porous silicon by anodic etching of a silicon wafer in aqueous acid solutions, such as a solution containing hydrofluoric acid. Thin layers of porous silicon 3 to 7 microns in depth are produced this way near the surface of the silicon wafer. Examples of such techniques are given in Kinney et al., U.S. Pat. No. 4,532,700 and Katoh et al., U.S. Pat. No. 5,156,896. This method of creating photoluminescing porous silicon creates problems for research and large scale production, since the process introduces various contaminants onto the surface of the silicon. Additionally, the process is environmentally detrimental because of the hazardous etching solutions required.
A non-etching method for production of photoluminescing porous silicon is disclosed in Hummel. et al., U.S. Pat. No. 5,397,429. This method involves the use of spark processing to create a porous surface on the silicon wafer. While successful in producing photoluminescing silicon, the process involves generating high voltage sparks which cannot always be controlled in a precise manner, making the process less suitable for use in precision situations. The technique cannot be used in situations where the high voltage and lateral extension of the sparking could damage other structures on the silicon substrate, such as on a computer chip already containing electronic circuitry.
It is an object of this invention to provide a method for producing photoluminescing porous silicon (n-type, p-type or undoped) or other semiconductor-type material which does not require nor utilize electrochemical etching in aqueous acid solutions and which can be controlled with some degree of precision to prevent extraneous damage during processing. It is an object to provide such a process which utilizes laser energy to produce the photoluminescing porous surface layer on the silicon substrate.